Method of fabricating a scalable nanoporous membrane filter

ABSTRACT

A method of fabricating a nanoporous membrane filter having a uniform array of nanopores etch-formed in a thin film structure (e.g. (100)-oriented single crystal silicon) having a predetermined thickness, by (a) using interferometric lithography to create an etch pattern comprising a plurality array of unit patterns having a predetermined width/diameter, (b) using the etch pattern to etch frustum-shaped cavities or pits in the thin film structure such that the dimension of the frustum floors of the cavities are substantially equal to a desired pore size based on the predetermined thickness of the thin film structure and the predetermined width/diameter of the unit patterns, and (c) removing the frustum floors at a boundary plane of the thin film structure to expose, open, and thereby create the nanopores substantially having the desired pore size.

I. FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The United States Government has rights in this invention pursuant toContract No. DE-AC52-07NA27344 between the United States Department ofEnergy and Lawrence Livermore National Security, LLC for the operationof Lawrence Liveimore National Laboratory.

II. FIELD OF THE INVENTION

The present invention relates to molecular filters and sieves, and moreparticularly to a method of fabricating a nanoporous membrane filterhaving arrays of uniform nanopores each scalable from a few nanometersto hundreds of nanometers in diameter for use in filtering, separating,sorting or otherwise screening molecules and particles.

III. BACKGROUND OF THE INVENTION

For applications such as protein screening, organic and inorganicmolecular development, and pre-screening of toxins and other moleculesfor sensor applications, there is a need for nanoporous filters andscreens with uniform pore size, scalable between ˜1-100 nm. For example,all protein production, isolation and purification efforts require thatthe proteins be: (a) separated away from other contaminating proteinsand other molecules, (b) analyzed to assess its degree of homogeneity,and (c) treated to change the type of solution or buffer in which it isdissolved. The fact that each protein behaves differently in each ofthese steps can often make the task of working with isolated proteinsdifficult, particularly when the goal is to develop high throughputmethods for their production and purification. Differences inhomogeneity following purification can be caused by variation inpost-translational modifications, dissociation of subunits, differencesin the degree of folding, and proteolytic degradation.

A variety of methods have been developed for determining the size of aprotein or protein complex, assessing the heterogeneity of thepopulation, or separating proteins from other molecules. For example,conventional methods for assessing the size of protein complexes haveincluded size exclusion chromatography (SEC) or electrophoresis innative gels, dynamic light scattering, electron microscopy, scanningprobe microscopy, sedimentation rates, mass spectrometry, nuclearmagnetic resonance (NMR) spectroscopy, neutron scattering, and smallangle X-ray scattering (SAXS). In addition to providing an estimate ofsize, several of these methods (SEC, sedimentation, dynamic lightscattering) have been used to directly or indirectly facilitate proteinseparation and purification. The development of many of these standardmethods for high-throughput applications in microfabricated systems,however, has remained difficult. Most methods currently used inhigh-throughput, chip-based systems involve electrophoretic separationsof the components. Many of these techniques also require costlyinstrumentation and are labor intensive.

And some common methods for characterizing the homogeneity of abio-molecule such as a protein or toxin are those that separate thecomponents based on physical size (e.g. size exclusion chromatography,mass spectrometry) or a combination of size and charge density (e.g. gelelectrophoresis). While all three techniques can be adapted for highthroughput applications and incorporated into automated systems, eachhas limitations. For example, size exclusion chromatography using gelmatrices dilutes the sample and has limited resolving power to provideaccurate details about size heterogeneity. Mass spectrometry can providethe most accurate assessment of sample homogeneity, but variations inionization efficiency can make it difficult to accurately quantify therelative proportion of the components. And electrophoretic methods canresolve molecules that differ by as little as a single positive ornegative charge, but apparently homogeneous samples can often containmultiple components that have the same charge density per unit mass.

Advances in the development of silicon and other materials withnanometer-scale (1-1000 nm) pores or slits have raised the possibilityof producing molecule sizing filters with a sufficiently large dynamicrange of size selection (extending from ˜1 nm to 1 μm) to cover, forexample, the entire range of known sizes of proteins and proteincomplexes. However, the use of standard lithographic processes forproducing the smallest of these features sizes (i.e. in the range of˜1-100 nm) has been difficult for large areas (i.e. greater than 1 cm²,and typically in the range of tens of cm²) required for most molecularfilter applications. And while non-lithographic methods have beendeveloped for producing near-nanometer pore sizes, their usefulness islimited due to lack of pore size uniformity and repeatability. Forexample, porous membranes created through anodic etching and mesoporoussilica formed through sol-gel process have non-uniform pore diameters,respectively, which typically vary over a broad range: ˜30-400 nm foranodic alumina and ˜2-20 nm in sol-gel films. These limitations aredifficult to address due to critical dependence of process chemistry onseveral variables such as solution concentration, temperature, andcurrent. Other filter materials such as zeolites have uniform pores, butonly in the relatively narrow range of ˜0.3-3 nm. Carbon nanotubes arebeing developed at the Lawrence Livermore National Laboratory forfilters in the 1-10 nm range, but scaling beyond this limit is extremelychallenging, and the cylindrical shape of the pores may presentadditional complications. Finally, ion-track etching throughpolycarbonate films can produce a wide range (−10 nm to ˜μm) of porediameters, but pore uniformity and flow rates have been observed to belimited to about ±20% and <0.1 mL/min/cm², respectively, for 10 nmdiameter pores.

Thus there is a need for a method of fabricating large-area nanoporousfilters and screens having uniform pores with scalable pore diametersranging from a few nanometers to hundreds of nanometers, and capable ofefficiently separating and characterizing molecules and small particles.

IV. SUMMARY OF THE INVENTION

One aspect of the present invention includes a method of fabricating ananoporous membrane filter having an array of uniform nanopores of adesired pore size, comprising: providing a thin-film multilayer having a(100)-oriented single crystal silicon layer adjacent a substrate layer,said single crystal silicon layer of a predetermined thickness whichetches anisotropically with an anisotropic wet chemical etchantselective to silicon over a material of the substrate layer; forming ahard mask layer over the single crystal silicon layer; depositingphotoresist over the hard mask layer; using at least twointerferometrically-arranged lasers to define in the photoresist an etchpattern comprising an array of unit patterns each having a predeterminedwidth that is substantially a function of the desired pore size and thepredetermined thickness of the single crystal silicon layer;transferring the etch pattern from the photoresist to the hard masklayer to expose select portions of an upper boundary surface of thesingle crystal silicon layer; anisotropically etching the exposed selectportions of the upper boundary surface of the single crystal siliconlayer with the anisotropic wet chemical etchant until a lower boundaryplane of the single crystal silicon layer is reached so that an array ofinverted frusto-pyramidal etch cavities are formed each having a frustumfloor at the lower boundary plane that is substantially equal in widthto the desired pore size; and etching a section of the substrate layerwith an etchant that is selective to the substrate material over siliconto remove the frustum floor of the etch pits at the lower boundary ofthe single crystal silicon layer and thereby form the array of unifoininanopores of substantially the desired pore size.

Another aspect of the present invention includes a method of fabricatinga nanoporous membrane filter having an array of uniform nanopores of adesired pore size, comprising: providing a thin-film multilayer having asubstrate and a thin film layer of predetermined thickness which etchesisotropically when etched with an isotropic wet chemical etchant that isselective to the thin film layer; depositing photoresist over the thinfilm layer; using at least two interferometrically-arranged lasers todefine in the photoresist an etch pattern comprising an array of unitpatterns each having a predetermined width that is substantially afunction of the desired pore size and the predetermined thickness of thethin film layer, wherein the etch pattern exposes select portions of thethin film layer; etching the exposed select portions of the thin filmlayer with the isotropic wet chemical etchant including controlling etchtime so that an array of frusto-spherical etch pits are formed eachhaving a frustum floor at a lower boundary of the thin film layer thatis substantially equal in diameter to the desired pore size; and etchinga section of the substrate with an etchant selective to the substrate toremove the circular frustum floor of the etch pits at the lower boundaryof the thin film layer and thereby form the array of uniform nanoporeshaving the desired pore size.

Another aspect of the present invention includes a method of fabricatinga nanoporous membrane filter having an array of uniform nanopores of adesired pore size, comprising: depositing photoresist on an uppermostlayer of a thin-film multilayer comprising a substrate and a thin filmlayer over the substrate, said thin film layer having a predeteiminedthickness and which etches with a wet chemical etchant that is selectiveto the thin film layer; using interferometric laser exposure to defineon the photoresist an etch pattern comprising an array of unit etchpatterns; etching the thin film layer with the wet chemical etchant sothat an array of frusto-geometric etch pits are formed each having afrustum floor at a lower boundary of the thin film layer that issubstantially equal in size to the desired pore size; and etching asection of the substrate with an etchant selective to the substrate toremove the frustum floor of the etch pits at the lower boundary of thethin film layer and thereby form the array of uniform nanopores havingthe desired pore size.

Generally, the present invention is directed to a method of fabricatingnanoporous membrane filters having uniformly sized and patterned 2Darrays of nanopores etched in a suitably rigid thin film layer of a typehaving a known etch profile when etched with a known compatible etchant(e.g. anisotropic etching profile of silicon using KOH). Examplematerials for the thin film layer include, for example, thin singlecrystal silicon or alternatively silicon dioxide or silicon nitride. Inany case, each pore of the arrays of nanopores is scalable from about1-100 nm diameter over a large area of about 1 cm² or greater. Thepresent invention also directed to the nanoporous membrane filters andmolecular sieves fabricated according to this method, for use infiltering, sorting, and otherwise screening molecules and particle.

In particular, the fabrication method of the present invention involvescreating uniform nanopores in a thin film structure (e.g. (100)-orientedsingle crystal silicon) which are produced by (a) creating an etchpattern comprising a plurality array of unit patterns usinginterferometric lithography, (b) using the etch pattern to etchfrustum-shaped cavities or pits in the thin film structure (which has apredetermined thickness) so as to control the dimension of the frustumfloor of each cavity to be substantially equal to a desired pore size,and (c) removing the frustum floors at a boundary plane of thefrustum-shaped cavities to expose, open, and thereby create the pores.

The membrane material in which the nanopore arrays are etched may beselected from various types of materials having a known etch profile forany number of etchants. Three example materials are discussed herein,including (100)-oriented single crystal silicon, SiO₂, and Si₃N₄. In anycase, the membrane material is provided as part of a thin-filmmultilayer which also includes a buffer layer adjacent to and boundingthe membrane material layer. Generally, the buffer layer may compriseany material that etches at a slower rate than the membrane materialwhen etched with an etchant selective to the membrane material. Forexample, a nitride or oxide such as silicon nitride or silicon dioxidemay be used as the buffer layer material when (100)-oriented silicon isetched with a wet chemical etchant selective to silicon over the siliconnitride or silicon dioxide, such as KOH. Also, while KOH is used hereinas a common example for anisotropically etching silicon, it isappreciated that other alternative etchants may be employed (e.g.tetramethylammonium hydroxide or “TMAH”) so long as they also havesimilar etch characteristics, such as selective etching andanisotropically or isotropically etching a particular type of material.

It is notable that a frustum is the portion of a solid (such as forexample a cone, pyramid, or sphere) which lines between two parallelplanes cutting it. As such, “frusto-pyramidal” means having the shape ofa frustum of a pyramid, “inverted frusto-pyramidal” means having theshape of a frustum of an inverted pyramid, and “frusto-spherical” meanshaving the shape of a frustum of a sphere. Also, “frusto-geometricshape” means having the shape of a frustum of a genericthree-dimensional geometric structure. It is also appreciated that“nanopore” is a nanoscale pore, “nanoporous membrane” is a membrane withnanoscale pores, and a “nanoporous membrane filter” is a filter having ananoporous membrane construction.

In a first exemplary embodiment, a nanoporous membrane filter isfabricated by anisotropically etching a nanopore array in a(100)-oriented single crystal silicon material. The silicon material isprovided as a layer of a multilayer substrate comprising the siliconlayer bounded by a silicon dioxide layer. A preferred example of such amultilayer substrate is a silicon-on-insulator (“SOI”) wafer. A siliconnitride or silicon dioxide layer is formed as a hard mask on the siliconlayer, and having an etch pattern that is itself patterned from thephotoresist etch pattern produced by interferometric lithography. Usingthe etch pattern of the hard mask as a template, the single crystalsilicon material is anisotropically etched using a wet etchant thatselectively etches silicon to SiO₂, such as KOH. Since the anisotropicetching of the (100)-oriented silicon material is known to produce(111)-oriented etch pit sidewalls angled at 54.47 degrees, thedimensions (i.e. width) of each unit pattern of the mask pattern and thethickness of the silicon layer are selected and predetermined so as toproduce a frustum floor at a boundary plane between the silicon layerand the silicon nitride/silicon dioxide layer having the desireddimension (width or diameter). By selectively etching the siliconnitride/silicon dioxide layer to silicon, the frustum floor is removedand the pore is exposed and opened.

In a second exemplary embodiment, a nanoporous membrane filter isfabricated by isotropically etching a nanopore array in thin film layerwhich is known to etch isotropically when etched with a wet chemicaletchant that is selective to the thin film layer. Example materials forthe thin film layer may include silicon nitride or silicon dioxide. Inthis case, the silicon nitride or silicon dioxide material is providedas a layer of a multilayer substrate comprising the silicon nitride orsilicon dioxide layer bounded by a silicon wafer layer. A photoresistpattern produced by interferometric lithography on the silicon nitrideor silicon dioxide layer also functions as the mask pattern for etching.Using the etch mask as a template, the silicon nitride or silicondioxide material is isotropically etched that selectively etches SiO₂ orSi3N4 to Si. Because the isotropic etch forms a rounded, substantiallyspherical cavity in the silicon nitride or silicon dioxide layer, it isappreciated that a bottom of the cavity tapers to a central nadir. Bycontrolling etch time, a frustum floor of the rounded etch cavity may beformed at a boundary plane between the silicon layer and the siliconnitride/silicon dioxide layer having a desired dimension (width ordiameter). By selectively etching the silicon to silicon nitride/silicondioxide, the frustum floor is removed and the pore is exposed andopened.

Because the method of fabrication of the present invention is based onhighly-developed lithographic processing methods used for semiconductorfabrication, it provides a key advantage in terms of its compatibilitywith large areas and simple processing tools, which can lead directly tolow manufacturing costs. It also enables certain advantages overnon-lithographic techniques. For example, pattern uniformity of the 2Dnanopore arrays of the present invention is determined by the wavelengthof the laser exposure source, which is an absolute constant. And poreuniformity is determined by etching anisotropy of <111> silicon planesrelative to the <100> silicon, which is highly predictable. And poresize itself is determined by processes parameters including the siliconfilm thickness, pattern periodicity, and oxidation, all of which arecontrollable to better than 1%. For example, nanometer-sized poreshaving average pore diameter of about 270 nm have been fabricated, withthe standard deviation of the pore area less than about 15% of theaverage area. Moreover, performance advantages of the nanoporousmembrane filters themselves include speed and efficiency of molecularmass transport, as well as reduced interference for transport by blockedpores.

The present invention is provided to solve the problem of purifying andcharacterizing bio-molecules. These arrays can enable a new class ofrobust, high-throughput, electronically-controlled filters forbio-molecule separation and synthesis, and for pre-screening ofmolecules in advanced sensors. Filter may be used in synthesizing andcharacterizing proteins. With the uniform pore arrays, basic questionsconcerning the structure of proteins and toxins, and their interactionswith other molecules, can be addressed with unprecedented efficiency toenable powerful sensors and advanced tools for proteomics.

The filter can be useful as a critical pre-concentration and rapidcharacterization component of chem/bio-sensors. For example, the filtercan be useful as a critical component of an advanced arsenic sensor, andin pre-screening of non-target materials for bio-sensors underdevelopment at LLNL. Generally, this filter would greatly improve arange of chem/bio sensor platforms, enabling a new class of rapid,electronically-controlled systems with very low alarm rates to bedeveloped. Enables a high-throughput protein characterization capabilitythat can be used to rapidly assess the size homogeneity and folded stateof an expressed or synthesized protein or protein complex. Finally, thefilter could be an important part of sensitive detectors for watercontaminants, such as arsenic.

V. BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and form a partof the disclosure, are as follows:

FIG. 1 is a schematic side view of a thin-film multilayer with a siliconnitride (or silicon dioxide) layer formed on a (100)-oriented singlecrystal silicon layer of the thin-film multilayer, in a first exemplaryembodiment of the method of fabricating a nanoporous membrane filter ofthe present invention.

FIG. 2 is a schematic side view of the thin-film multilayer of FIG. 1additionally having a photoresist layer deposited on the silicon nitride(or silicon dioxide) layer 1.

FIG. 3 is a schematic side view of the thin-film multilayer of FIG. 2being exposed to interferometrically-arranged lasers to define an etchpattern on the photoresist.

FIG. 4 is a perspective view of the nanoporous membrane filter followingthe interferometric laser exposure of FIG. 3 and showing the etchpattern created as a plurality array (11×11) of unit patterns in thephotoresist.

FIG. 5 is a schematic side view of the thin-film multilayer of FIG. 4being plasma etched to transfer the etch pattern to the silicon nitride(or silicon dioxide) layer to be used as a hard mask layer. For clarity,only a single representative unit pattern is shown here.

FIG. 6 is a schematic side view of the thin-film multilayer with theetch pattern produced from FIG. 5 and further illustrating ananisotropic etching step using a wet chemical etchant such as KOH thatanisotropically etches silicon. Similar to FIG. 5, a singlerepresentative unit pattern is shown here for clarity.

FIG. 7 is a schematic side view of the formation of an array of inversefrusto-pyramidal etch pits (a single representative etch pit shown)produced by the KOH anisotropic wet etching step of FIG. 6.

FIG. 8 is a perspective view of the nanoporous membrane filter followingthe plasma etch of FIG. 5 and showing the etch pattern comprising aplurality array of unit patterns in the hard mask layer of the siliconnitride or silicon dioxide.

FIG. 9 is a perspective view of the nanoporous membrane filter of FIG. 7showing the array of inverse frusto-pyramidal etch pits as seen throughthe etch pattern of the silicon nitride or silicon dioxide hard masklayer.

FIG. 10 is a perspective view of the nanoporous membrane filter of FIG.9 after removing the silicon nitride or silicon dioxide hard mask layer(optional) to provide an unobstructed view of the array of inversefrusto-pyramidal etch pits.

FIG. 11 is an enlarged view of the single representative inversefrusto-pyramidal etch pit shown in FIG. 7.

FIG. 12 is a schematic side view of the nanoporous membrane filterfollowing FIG. 7 and showing the nanopores created at the bottom of theinverse frusto-pyramidal etch pits by removing a portion of the siliconhandle wafer by selectively etching silicon over SiO₂, and subsequentlyselectively etching the SiO₂ over silicon.

FIG. 13 is an underside perspective view of the nanoporous membranefilter of FIG. 12 showing the exposed and opened nanopore array.

FIG. 14 is a schematic side view of a thermal oxidation step in anexemplary embodiment of the fabrication method, for decreasing pore sizeand/or providing voltage control.

FIG. 15 is a schematic side view of flow through an exemplary embodimentof the nanoporous membrane filter of the present invention having ametal deposited on the lower surface of the top silicon layer.

FIG. 16 is a schematic side view of a thin-film multilayer having asilicon nitride layer formed on (or a silicon dioxide layer is formedon) a silicon substrate, followed by the deposition of photoresist, in asecond exemplary embodiment of the method of fabricating a nanoporousmembrane filter of the present invention.

FIG. 17 is a schematic side view of the thin-film multilayer of FIG. 16being exposed to interferometrically-arranged lasers to define an etchpattern on the photoresist.

FIG. 18 is a schematic side view of the thin-film multilayer with theetch pattern produced from FIG. 17 and further illustrating an isotropicetching step using a wet chemical etchant such as for examplehydrofluoric acid (HF) that isotropically etches silicon dioxide. Forclarity, a single representative unit pattern is shown.

FIG. 19 is a schematic side view of the formation of an array of rounded(spherical) cavities produced by the isotropic wet etching step of FIG.18.

FIG. 20 is a schematic side view of the nanoporous membrane filterfollowing FIG. 19 and showing the nanopores created at the bottom of therounded (spherical) cavities after removing a portion of the siliconhandle wafer by selectively etching silicon to Si₃N₄ or SiO₂.

FIG. 21 is a schematic side view of a metal deposition step followingFIG. 20 to further control pore size and/or provide voltage control.

VI. DETAILED DESCRIPTION

Turning now to the drawings, FIGS. 1-13 show a first exemplaryembodiment of the method of fabricating a nanoporous membrane filter ofthe present invention. In this first embodiment, an array of uniformnanopores is etch-formed in a suitably rigid thin-film layer of(100)-oriented single crystal silicon which forms the main body of thenanoporous membrane filter. As such, this thin-film layer may also becharacterized as the “membrane layer,” “membrane body,” or “membranestructure.”

Generally, the (100)-oriented single crystal silicon is provided as atop layer of a thin-film multilayer which also includes one or moreunderlying substrate layers. In FIG. 1, the thin-film multilayer isparticularly shown as a silicon-on-insulator (“SOT”) wafer, generallyindicated at 10. The SOI wafer 10 includes a (100)-oriented singlecrystal silicon top layer 13 and a silicon base layer 11 (e.g. a siliconwafer substrate for handling the SOT) as the two outer layers, and abuffer layer 12 between the two outer layers. It is notable that thebuffer layer 12 is adjacent to and bounds the (100)-oriented singlecrystal silicon top layer 13, such that a lower boundary plane of thetop layer is located between the top layer and the buffer layer. It isalso notable that with respect to the top layer, both the buffer layer12 and the silicon base layer 11 may be characterized together as thesubstrate, and individually as substrate layers. The single crystalsilicon top layer 13 of the SOI wafer is typically thin and on the orderof tens of nm to tens of microns thick, while the silicon base layer 11(“handle) is typically hundreds of microns thick. The buffer layer 12may be any suitable material having a slower etch rate than silicon whenan etchant specific to silicon is used. For example, because the toplayer is (100)-oriented single crystal silicon, silicon nitride orsilicon dioxide may be used for the buffer layer 12.

An important parameter of the single crystal silicon layer is thicknessbecause it is one of several key variables used in the present inventionto control a desired pore size of the nanoporous membrane filter. Inparticular, it is important that this layer have a highly uniformthickness over large areas in order to create pores of a uniform size.In this regard, SOI wafers are useful because large-area SOI wafers ofexcellent quality and thickness uniformity are readily available from anumber of sources. For example, the standard deviation in thickness of acommon commercially-available 70-nm thick silicon-on-insulator layer isless than the thickness of a single atomic layer over 1 cm². And(100)-oriented 300 mm diameter silicon wafers are also commerciallyavailable where the top silicon layer is between 0.34 μm to 0.6 μm andthe silicon thickness standard deviation is 1.7 nm over the entire waferarea.

FIG. 1 also shows the deposition of a hard mask layer 14 on the singlecrystal silicon top layer 13. Example materials for the hard mask layerinclude silicon nitride or silicon dioxide. Following the hard maskdeposition, photoresist 15 is next formed over the hard mask 14 as shownin FIG. 2. The uniformity of the photoresist thickness can be made to bevery high even over the surface of a wafer 10 cm in diameter or larger.

Next, FIGS. 3 and 4 show the use of interferometric lithography todefine etch patterns having a predetermined width or diameter. As shownin FIG. 3, two or more continuous wave coherent laser beams (e.g. 16,17) are simultaneously incident on a surface coated with a thin film ofphotoresist 15, such that the intensity at the surface is modulated bythe interference pattern. It is notable that pattern uniformity of the2D arrays is determined by the wavelength of the laser exposure source,which is an absolute constant. FIG. 4 is a perspective view of thenanoporous membrane filter showing an exemplary etch pattern producedand developed as a plurality array 18 of unit patterns 19 on thephotoresist layer 15. As shown in FIG. 4, each unit pattern reveals aportion 20 of an upper boundary surface of the underlying hard masklayer 14. It is appreciated that interferometric lithography makes useof domains of constructive interference between two coherent wavesresulting in a 1-D periodic pattern defined by λ/2 sin θ, where λ is thelaser wavelength and 2θ is the angle between two beams. By addingmultiple exposure processes, arbitrary patterning is possible althoughstill within a repetitive unit cell. By combining with nonlinear spatialperiod division techniques, patterning resolution beyond the theoreticalλ/2 limit has also been demonstrated. Interferometric lithographytechniques may be used in creating openings of ˜150-nm width range at aperiod of 1.3 and 0.7 μm, and trench widths smaller than the period byas much as factors of 10 are also achievable. For example, parallellines with widths as small as 135 nm can be easily created by applyingtwo 257-nm laser beams incident at 80 degrees angular separation on aplanar substrate. In tests performed at Lawrence Livermore NationalLaboratory, the uniformity of the photoresist patterns produced byinterferometric lithography was shown to be superior than for PCTE. Inparticular, the standard error (average divided by standard deviation)of the pattern diameters was 0.76 for PCTE vs. 0.15 for interferometriclithography.

In FIG. 5 the thin-film multilayer is next shown being plasma etched 21to transfer the etch pattern, as represented by unit pattern 19, to thehard mask layer 14. The patterned hard mask layer 14 is shown in FIG. 8showing a plurality array 21 of unit patterns 22, with each unit patternexposing select portions 23 of an upper boundary surface of the singlecrystal silicon top layer 13. The patterned hard mask layer 14 of FIG. 8is now ready for etching the single crystal silicon top layer.

Using the patterned hard mask 14 as a template, FIG. 6 shows the singlecrystal silicon top layer 13 being anisotropically etched using a wetetchant 24 that selectively etches silicon over SiO₂, such as forexample KOH. In particular, KOH preferentially etches the (111) planesof silicon. And FIGS. 7, 9 and 10 show the formation of a pluralityarray 21 of inverse frusto-pyramidal etch pits 26 produced by the KOHanisotropic wet etching step of FIG. 6. In particular, FIG. 9 shows thearray of inverse frusto-pyramidal etch pits 26 as seen through thepatterned hard mask layer 14 and the array 21 of unit patterns 22, whileFIG. 10 shows the array 25 of etch pits 26 with the hard mask layerremoved (which is not necessarily required in the present fabricationmethod). In this way, inverted frusto-pyramidal cavities are formed inthe single crystal silicon, and the intersection of the apex of thesepyramids with the underlying silicon dioxide layer of thesilicon-on-insulator structure defines the size of the pores. Theanisotropic etch can create near-geometrically perfect structures. Toachieve size uniformity among the pores created at the bottom of theinverse frusto-pyramidal cavities, the main technological requirement isthat the dimensions of the mask be defined reproducibly in silicon. Inother words, pore size uniformity is dependent on the dimensionaluniformity amongst the unit patterns produced by interferometriclithography and the thickness uniformity of the single crystal silicontop layer.

And FIG. 11 is an enlarged view showing details of the inversefrusto-pyramidal etch cavity/pit 26 shown in FIG. 7 formed by theanisotropic etching step. Etching of single crystal silicon inorientation-dependent alkaline solutions (e.g. KOH) is very wellcharacterized. The etch anisotropy of the alkaline solutions resultsfrom the fact that there are fewer surface Si—OH bonds per unit cell on(111) compared to (100) and (110) surfaces, leading to higher energy tobreak the back bonds of the (111) surface silicon atoms. Thisanisotropic effect is employed to generate V-grooves in (100) siliconwafers using openings in an appropriate mask such as an oxide layer. The(111) planes form an angle of 54.74° with the (100) plane. Each inversefrusto-pyramidal etch cavity shown in FIG. 10 has four (111) planesurfaces each forming an angle of 54.47 degrees with the (100) plane anda frustum floor at a lower boundary plane of the single crystal siliconlayer that is substantially equal in width to the desired pore size.Since the anisotropic etching of the (100)-oriented silicon material isknown to produce four (111)-oriented etch pit sidewalls angled at 54.47degrees, the dimensions (i.e. width, w₁, in FIG. 11) of each unitpattern of the etch pattern and the thickness of the silicon layer areselected and predetermined so as to produce a frustum floor at aboundary plane between the single crystal silicon layer 13 and thesilicon nitride/silicon dioxide layer 12 having the desired pore sizedimension (i.e. w₂ in FIG. 11). The fabrication of nanopores issufficiently deterministic, and may be approximated by the followingmathematical formula:

${{pore}\mspace{14mu} {width}},{w_{2} = {{pattern}\mspace{14mu} {width}}},{w_{1} - {\left( \frac{2}{\tan \; \theta} \right){Si}\mspace{14mu} {thickness}}},d$

where W₁ is the lithographically defined etch pattern width; d is thesilicon thickness, and w₂ is the pore width/diameter. And Bis the angleof the (111)-oriented cavity walls, which is 54.47 degrees. Otherfactors not included in the above approximation is the total etch time,as well as the etch rate in the (111) plane which is relatively slowcompared to etching in the (100) and (110) planes but is notnon-existent. This causes some amount of undercutting below thepatterned hard mask, which can produce a non-negligible effect on thepattern width value, w₁. For the case where w₂=0 (appropriate for ananometer-scale pore), d=w₁/1.414. Therefore, for w₁=1 μm, the siliconfilm thickness must be ˜0.7 μm to achieve a perfect V-groove (w₂=0) in1D, or pyramidal structure, in 2D. This example shows thatnanometer-scale trenches/holes can be achieved with a silicon film 0.7μm thick, with micrometer-scale lithography.

FIGS. 12 and 13 show the final step of filter fabrication, where asection of the handle wafer 11 and buffer layer 12 are selectivelyetched away. Consequently, the frustum floors at the bottom of the etchcavities are thus removed to expose, open and thereby create thenanopore “windows” through the membrane structure so that fluids canpass through the pore and the inverted frusto-pyramid cavity. FIG. 12 isa schematic side view of the nanoporous membrane filter after removing aportion of the handle wafer by selectively etching silicon to SiO₂, andsubsequently selectively etching the buffer SiO₂ layer to silicon (andexposing lower surface 27 in FIG. 12), so as to expose and open thenanopores at the bottom of the pyramidal etch pits. In FIGS. 12 and 13,an open volume 28 is shown formed leading into the array 30 of nanopores29 thus opened and formed. And FIG. 13 is an underside perspective viewof the nanoporous membrane filter of the present invention after thenanopores are exposed and opened following FIG. 12. The nanopores thusformed are supported in a thick, mechanically robust silicon frame,allowing for direct integration of the filter with macroscopicmicrofluidic components. Using standard expressions for stress developedin supported diaphragms it has been calculated that a load of 40 kPa canbe readily sustained by a membrane which is 250 nm thick, 20 μm indiameter.

After formation of the general nanoporous membrane filter is complete,the pore sizes may optionally be further “tuned” with angstrom-levelprecision, as shown in FIG. 14, via thermal oxidation of silicon toproduce a SiO₂ layer, indicated at 31. This reduces the pore size toimprove size resolution, which is shown as w₃ in FIG. 14. Pore sizereduction tuning may also be followed by partial removal of the SiO₂layer (using HF) to again increase pore size. Removing the resultingSiO₂ with HF acid will increase the pore size, while leaving SiO₂ inplace will decrease the pore size, since the thickness of the consumedsilicon is 44% of the thickness of the oxide formed. This approach iseffective since the oxidation rate of silicon slow and well-understood.For example, it has been known that for (111) oriented silicon at 700°C., oxide forms at a rate of less than 1 nm/hour.

FIG. 15 also illustrates an example flow direction of the completedmembrane filter. Note that the flow direction during filter operationsends unfiltered fluid to the planar exterior surface of the pore,minimizing fouling. The tapered geometry of the pore facilitates highflow rates since the constriction volume is localized at this planarsurface, as opposed to being extended through the thickness of themembrane as would be true for a cylindrical pore. This approach canachieve similar pore densities as track-etched pores in polycarbonatemembranes, and if flow rate scales with pore aspect ratio, up to −100×improvements can be realized. The tapered cavity geometry of the presentinvention is preferred to maximize the dynamic range of achievable flowrates, and especially to allow for effective separation at high flowrates needed for massively parallel bio-molecule purification anddetection schemes.

In addition, FIG. 15 also illustrates the optional step of depositing ametal layer 32 on the lower surface of the top silicon layer 13 forvoltage control. Voltage controlled pores allow for greatly enhancedselectivity. The nanopores of the present invention are uniquely suitedfor use in a powerful method for voltage control of bio-molecule flowrates that can improve purification efficiency by more than ahundred-fold. For filtration and sensing of bio-molecules, high levelsof selectivity can be achieved at the pore opening by electricallybiasing the surface of the pore while matching the pH of the workingsolution to the isoelectric point of the target molecule. At pH valuesabove and below the isoelectric point, proteins experience Coulombicinteractions with a charged pore surface, whereas for pH values equal topI these forces disappear and transport is greatly enhanced. Voltagecontrol can be achieved by growing a thin thermal oxide on the Si, thendepositing a metal gate layer such as Au. Alternately, the silicon canbe doped to allow direct voltage control of the surface, which could beespecially advantageous for small ˜1-10 nm pores where the roughness ofa deposited metal layer would introduce variability in the effectivefield at the opening of the pore. In the tapered pore geometry of thepresent invention, voltage control is confined to the narrowest part ofthe pore. This architecture allows for precise mass transfer control atthe apex while the broadening of the pore beyond the apex gives minimalmass transfer resistance.

Chemical functionality may also be designed into the nanopores of thepresent invention. In particular, additional chemical functionality canbe imparted to an Au surface through thiol-functionalized molecules, orto silicon through direct silicon-carbon bond formation (or silanereactions with SiO₂) and other related methods. Coupled with thehigh-throughput tapered geometry of the voltage-controlled nanopore,these coatings can improve selectivity, further reducing and eveneliminating issues associated with fouling.

FIGS. 16-21 show a second exemplary embodiment of a method offabricating a nanoporous membrane filter of the present invention, wherethe membrane is a silicon nitride or silicon dioxide material. Inparticular, FIG. 16 is a schematic side view of a thin-film multilayerhaving a silicon nitride layer 41 formed on (or a silicon dioxide layeris formed on) a silicon substrate 40, followed by the deposition ofphotoresist 42. Next, in FIG. 17 the photoresist 42 is exposed to atleast two interferometrically-arranged lasers (e.g. 43, 44) to define anetch pattern on the photoresist, shown by single representative unitpattern 45 in FIG. 18. In FIG. 18, an isotropic etching step using a wetchemical etchant 46 such as for example hydrofluoric acid (HF) is usedto isotropically etch the silicon dioxide layer 41. In this secondembodiment, the silicon dioxide layer (or silicon nitride layer) is usedas the suitably rigid membrane structure of the nanoporous filter. Thisis shown in FIG. 19 showing the isotropic formation of an array ofrounded (spherical) cavities 47 in the silicon nitride layer 41 producedby the isotropic wet etching step of FIG. 18. The etch cavity producedin this manner is a frusto-spherical cavity since both the top andbottom planes are flat. In particular, the frustum floor of thefrusto-spherical cavity is shown having a size w₂, which is the desiredpore size of the filter. In this case, the dimensions of the frustumfloors, and thus the pore sizes, can be tuned by controlling etch time.Similar to the first embodiment, the nanopores are created by removing aportion of the silicon substrate 40, so as to remove the frustum floor.This is shown in FIG. 20 after removing a portion of the silicon handlewafer by selectively etching silicon over Si₃N₄ or SiO₂. An open volume49 is created bounded in part by a lower surface 48 of the membranestructure, and leading to the nanopores 50. And FIG. 21 shows aschematic side view of an optional metal deposition step, indicated at51, to further control pore size and/or provide voltage control. Thetuned pores are indicated at 50′.

While particular operational sequences, materials, temperatures,parameters, and particular embodiments have been described and orillustrated, such are not intended to be limiting. Modifications andchanges may become apparent to those skilled in the art, and it isintended that the invention be limited only by the scope of the appendedclaims.

We claim:
 1. A method of fabricating a nanoporous membrane filter havingan array of uniform nanopores of a desired pore size, comprising:providing a thin-film multilayer having a (100)-oriented single crystalsilicon layer adjacent a substrate layer, said single crystal siliconlayer of a predetermined thickness which etches anisotropically with ananisotropic wet chemical etchant selective to silicon over a material ofthe substrate layer; forming a hard mask layer over the single crystalsilicon layer; depositing photoresist over the hard mask layer; using atleast two interferometrically-arranged lasers to define in thephotoresist an etch pattern comprising an array of unit patterns eachhaving a predetermined width that is substantially a function of thedesired pore size and the predetermined thickness of the single crystalsilicon layer; transferring the etch pattern from the photoresist to thehard mask layer to expose select portions of an upper boundary surfaceof the single crystal silicon layer; anisotropically etching the exposedselect portions of the upper boundary surface of the single crystalsilicon layer with the anisotropic wet chemical etchant until a lowerboundary plane of the single crystal silicon layer is reached so that anarray of inverted frusto-pyramidal etch cavities are formed each havinga frustum floor at the lower boundary plane that is substantially equalin width to the desired pore size; and etching a section of thesubstrate layer with an etchant that is selective to the substratematerial over silicon to remove the frustum floor of the etch pits atthe lower boundary of the single crystal silicon layer and thereby formthe array of uniform nanopores of substantially the desired pore size.2. The method of claim 1, wherein the substrate includes a silicon waferand a buffer layer separating the single crystal silicon layer from thesilicon wafer.
 3. The method of claim 2, wherein the thin filmmultilayer is a silicon-on-insulator (SOI) wafer comprising the siliconwafer, the single crystal silicon layer, and an insulating buffer layer.4. A method of fabricating a nanoporous membrane filter having an arrayof uniform nanopores of a desired pore size, comprising: providing athin-film multilayer having a substrate and a thin film layer ofpredetermined thickness which etches isotropically when etched with anisotropic wet chemical etchant that is selective to the thin film layer;depositing photoresist over the thin film layer; using at least twointerferometrically-arranged lasers to define in the photoresist an etchpattern comprising an array of unit patterns each having a predeterminedwidth that is substantially a function of the desired pore size and thepredetermined thickness of the thin film layer, wherein the etch patternexposes select portions of the thin film layer; etching the exposedselect portions of the thin film layer with the isotropic wet chemicaletchant including controlling etch time so that an array offrusto-spherical etch pits are formed each having a frustum floor at alower boundary of the thin film layer that is substantially equal indiameter to the desired pore size; and etching a section of thesubstrate with an etchant selective to the substrate to remove thecircular frustum floor of the etch pits at the lower boundary of thethin film layer and thereby form the array of uniform nanopores havingthe desired pore size.
 5. The method of claim 4, wherein the thin filmlayer is selected from a group consisting of SiO₂ and Si₃N₄.
 6. A methodof fabricating a nanoporous membrane filter having an array of uniformnanopores of a desired pore size, comprising: depositing photoresist onan uppermost layer of a thin-film multilayer comprising a substrate anda thin film layer over the substrate, said thin film layer having apredetermined thickness and which etches with a wet chemical etchantthat is selective to the thin film layer; using interferometric laserexposure to define on the photoresist an etch pattern comprising anarray of unit etch patterns; etching the thin film layer with the wetchemical etchant so that an array of frusto-geometric etch pits areformed each having a frustum floor at a lower boundary of the thin filmlayer that is substantially equal in size to the desired pore size; andetching a section of the substrate with an etchant selective to thesubstrate to remove the frustum floor of the etch pits at the lowerboundary of the thin film layer and thereby form the array of uniformnanopores having the desired pore size.